H-bomb or not, experts say North Korea near its nuclear goal

North Korea's latest nuclear test was part theater, part propaganda and maybe even part fake. But experts say it was also a major display of something very real: Pyongyang's mastery of much of the know-how it needs to reach its goal of becoming a full-fledged nuclear state.

It remains unclear whether North Korea tested, as it claims, a hydrogen bomb ready to be mounted on an ICBM.

But Sunday's test, the sixth and most powerful North Korea has conducted since its first in 2006, was a stunning advance in its demonstrated ability to build high-yield nuclear weapons. The explosion is believed to have ranged from 140 kilotons to potentially double that — or more — if it was conducted at a greater depth than has been calculated.

The power of the blast is important.

It will likely prove to be at least 10 times stronger than anything the North tested before. That's an important indicator of whether the device was the hydrogen bomb North Korea says it was.

H-bombs, more formally called thermonuclear devices, date to the 1950s and have the potential to be far stronger than simpler fission bombs like those used on Hiroshima and Nagasaki by the United States at the end of World War II. The biggest ever, nicknamed "Tsar Bomba," was detonated by the Soviet Union in 1961. It had the explosive power of 50 megatons, the equivalent of 3,800 Hiroshima bombs.

A yield estimate in the 100-kiloton range would put North Korea's test at the borderline for what is expected from a thermonuclear device. The higher estimates of 200-plus that are being offered by some experts are more in line with an H-bomb.

Just ahead of the test, North Korea's state-run media released photos showing leader Kim Jong Un surrounded by the country's top nuclear scientists inspecting what it called a two-stage thermonuclear weapon. The size and peanut-like shape of the weapon fits relatively well with known designs that could plausibly be bolted into a re-entry vehicle and paired with a ballistic missile capable of reaching the U.S. mainland.

But propaganda photos demand caution.

"People assume that's what was tested, but who knows," said physicist David Wright of the Union of Concerned Scientists. He said it is impossible to say at this point how big the test bomb was, or which of many possible bomb designs was used.

If radiation leaked from the test, military "sniffer" aircraft designed to pick up radioactive signatures could help the U.S. and its allies determine what the North actually tested.

"With luck, reports of a collapse of the (testing) tunnel may mean there is some venting, which would give useful information," Wright said.

David Albright, a former U.N. weapons inspector in Iraq and founder of the Institute for Science and International Security, said he also remains unconvinced that the North has an H-bomb, or that it detonated one Sunday.

"North Korea understands our fears and I believe the object in the picture was a model meant to play on those fears, sow division, and bolster their deterrent," he said.

Even so, he said North Korea could well be seeking to develop two-stage weapons because their explosive yield can be much higher than other designs; the requirement for fissile material is less; and their elongated shape can potentially fit more easily into missile re-entry vehicles.

"I think North Korea needs more tests and time to develop a miniaturized two-stage thermonuclear weapon," he said. Still, he said Sunday's test has prompted him to move up his estimate of when it will have such a weapon, which he now puts at less than two years away.

Other experts were more willing to guardedly accept the North's H-bomb claim.

"Their statements about missile and nuclear tests have become increasingly detailed, precise, and credible," said Joshua Pollack, a senior research associate at the Middlebury Institute of International Studies in Monterey, California. "They've shown us more and more details in the process. ... So context alone suggests they're telling the truth about this."

Though the superpowers conducted them with alarming frequency in the 1950s and '60s, North Korea is the only country in the world that still carries out underground nuclear tests.

Its first was something of a fizzle, with an explosion that was only about 1 kiloton. The second, three years later, was much bigger, fixed some of the problems with the first device and set the stage for a more ambitious test in 2013, which was the first ordered by Kim Jong Un. The fourth and fifth tests were both conducted last year, with what the North claimed was its first H-bomb in January and then a more powerful device in September.

Pollack believes there could be more — possibly many more — underground tests ahead.

"New designs and major new features can always use testing. The U.S. did it over 1,000 times! Plus, it's a stick in the eyes of the U.S."

He noted the North has four tunnel openings into four mountains at their Punggye-ri test site in its mountainous northeast. The first tunnel was abandoned after just one use, probably because gases leaked and authorities realized the geology there was unfavorable. They conducted their next five tests, including Sunday's, at the second mountain. The others remain unused.

Albright, the former arms inspector, estimates North Korea needs about 2 kilograms of plutonium and 6 to 10 kilograms of weapons-grade uranium to achieve a composite core bomb with a 10-kiloton yield. Based on its known production capabilities, he says, it has about 33 kilograms of separated plutonium and 175 to 645 kilograms of weapons-grade uranium.

That's not enough for a vast nuclear arsenal, but it is enough to steadily expand the arsenal and conduct many more blasts at Punggye-ri.

"I do expect more tests in the absence of any serious dialogue with the U.S.," Pollack said, "although one might say that this is a form of dialogue.",